I've tested this lens on 3 focal lenghts, 24, 50, and 105 mm. Analysis at the 24 mm at various f stops is given here. It is sharp in all the apertures, the best being f/8. Dependence of the image quality in various parts of the full 24x36mm frame is shown below, with the average resolution in hundreds of line widths per picture height (LW/PH) given below each chart.

Analysis at 24 mm.
Even at the full aperture there is a remarkable sharpness throughout the entire frame. Only at the very edge of the frame the picture quality drops below 3000 LW/PH. For a standard digital SLRs the frame corner is around the 12th mm, so that the lens is almost uniformly sharp.
At the optimum aperture, f/8, the lens performance is excellent throughout the entire frame (resolution of "37" is the max. theoretical number in this test). There is about 2.3% reduction of the circle size at the sides of the frame compared to the circle size at the center. This is much better than in the EF 24mm f 1.4 lens. Notice the roundness of the circles at the sides. Distortion is remarkably low: the circle becomes a barely distinguishable ellipse (major axis length divided by minor axis length differs by only 0.5%). Chromatic aberration is visible, but not large for this focal length.

Analysis at the 50 mm at various f stops is given here. The lens is the sharpest at this focal length. The best performance is at f/8. Even at the full aperture there is a remarkable sharpness throughout the entire frame.
At the optimum aperture, f/8, the lens performance is excellent throughout the entire frame (resolution of "37" is the max. theoretical number in this test). This is the only lens that I've tested with a constant maximum resolution of 3700 LW/PH throughout the entire full frame. There is a surprisingly large, 5.4% increase of the circle diameter at the sides of the frame compared to the circle size at the center. At the sides the circles look elongated slightly, with a 5.3% difference in extreme axis. There is practically no chromatic aberration.

Analysis at the 105 mm at various f stops is given here. It has a remarkably uniform sharpness throughout the entire frame at all the apertures. At this focal length the lens sharpness is very good, but it is the the lowest than at the other two focal lengths tested.
The size of the circles remain pretty much constant trhoughout the frame except at the very sides (over 15 mm), where the circles size increase to 3.6%. At the 21st mm the circles are slightly elongated, with a 3.8% difference in extreme axis. Chromatic aberration may be a matter of concern with the full frame cameras, but negligible compared to super-zoom lenses.

This is an excellent Canon zoom lens. Vignetting is not a problem, especially for standard digital SLRs. This is to be expected when its brightness is not on the exceptional side.